rsj - preventing thermal bridge

[endecotp]

Aerogel is no more fragile than mineral wool.
The vacuum panels are fragile, and also significantly better insulators than aerogel.
Both are expensive. I've never seen an actual price for vacuum panels. 10mm aerogel is about £50/m^2 in small quantities.

 

[John D v2.0]

AIUI they're quite similar in most regards; lambda, cost, fragility..

(Edit: corrected lambda figures thanks to endecotp)
Lambda is 0.007 as opposed to 0.015, cost, I can't even work out the price but I guess way more expensive! And fragility I don't want to find out.
Either way it's a surprising thing for an architect to completely forget to mention until asked for a detailed section drawing of that area.

 

[endecotp]

Rockwool: 0.035 W/mK
Polystyrene: 0.035 W/mK
PIR insulation board: 0.022 W/mK
Phenolic insulation board: 0.018 W/mK
Spacetherm Aerogel: 0.015 W/mK
Optim-R vacuum panel: 0.007 W/mK

Aerogel is not much better than the best "conventional" insulation boards; it only makes sense if you need a product that has some of the properties of mineral wool, i.e. it is flexible and vapour-open.

(Yes, off-topic...)

 

[John D v2.0]

Thanks endecotp
I'm a lot more calm today and i didn't lose as much sleep! I hope there will be a happy resolution - Monday I will be meeting the SE and speaking with the senior architect so hopefully some things will get done.
Any more advice (or agreement) on the beam issue would be very kind in case I need to debate things robustly with the architect!

 

[cjard]

Aerogel is not much better than the best "conventional" insulation boards; it only makes sense if you need a product that has some of the properties of mineral wool, i.e. it is flexible and vapour-open.

(Yes, off-topic...)

Yes, though VIPs are often just aerogel, in a bag with all the air sucked out.. once the vacuum is lost, you have.. aerogel!

0.007 is the conductance of a panel when you take the thermal bridges of the edges into account. Dunno how they get away with saying that it's 0.007 after it ages(which I take to mean, the vacuum has failed / degraded in service)

Regardless, the price performance ratio of both is insane compared to regular EWI materials and I cannot see why they can't be employed here.. vips and aerogels are employed when there isn't the space for anything better value but John certainly has the space, especially if it ain't built yet!

 

[John D v2.0]

especially if it ain't built yet!

Yes! Well half built, but the architect doesn't want to solve things, rather pretend they intended it all along.
Sorry for drip feeding, just for the avoidance of doubt, this is how built it is right now. The windows haven't yet been ordered. Click to zoom in.

 

[cjard]

Gotta say, there ain't a huge amount of difference between what your architect has specced and the Velfac detail.. window position looks to be about it!

I had two lintels (three actually but the outer pair were bolted together) which made some things easier, especially as they were at different heights
.

I think your biggest niggle is a single lintel spanning the cavity.. maybe lift your lintel more and have the window fixings on a steeper angle. They're only lateral restraint. You can pack and joist hanger the ceiling joists if you need.. by increasing the height of the lintel you increase the amount of insulation you can squeeze in, so you can move away from these fancy techs and to somehing that will withstand a bit of punishment.

Of course you can also shorten your window to suit too.. depends on how tall you want them vs the ballache of reconfiguring the lintel

 

[John D v2.0]

Thanks cjard, that's really useful info about cranking the fixings, I'll talk to the window fitter about that. There's not really any leeway to get the window height we want, however we can ask them to lift the steel a little and redo the pad stones.

So in your case you have one of the steel(s) on the cold side and the other on the warm side.
In my case I'd be trying to get the whole steel mostly on the warm side, right?
That would handle the interstitial condensation question better I think.

 

[cjard]

pretty much, though the outer pair of steels are cold side and the inner single in the timber frame bit is warm side. The window connects to 18mm ply that is hilti gunned onto the outer steel. Technically the ply is hence the coldest bit.. it shouldn't lose heat to the cavity, but it will lose it to the world. I'm not too bothered about this because a foot wide strip of ply with the narrow edge facing the room is going to transmit less heat than an alu window frame, even if the frame is thermally broken

There comes a point where you have to stop worrying about these things. If you're newvuilding a passivhaus that gets all its winter heat from a couple of hours worth of sun streaming through the carefully sized front windows and hitting a wall that has to be a particular colour or else.. then fine, but for the rest of us, the best efforts we go to may cost so much that we never recoup the outlay on the heating bills. Getting too overwrought on the details is a theory exercise rather than a practical one and the heat loss is an extra few hundred kilowatt hours a year. It's still better than anything you've ever lived in before and environmentally the right thing to do so our children stand a chance of inheriting a world that's turned to **** in some other way instead! it's also, I feel, a bit bows n arrows against the lightning, as we'd have to complete a passivhaus refit of one home every minute to reach the 2050 carbon reduction goals.. or something like that. Bet that ain't happening!

Make sure your steel isnt leaking heat elsewhere that you haven't been as focused on, because this bit has taken all the attention.. things like the ends that span the cavity, as the cavity is cold

 

[John D v2.0]

Thanks again, and actually I'm more worried about condensation than specifically the thermal performance. In the existing 1920s house we get a bit in colder areas, but at least it all performs terribly so the condensation generally spreads out and evaporates quickly. If the rest of the extension is well insulated, the reveal will always be damp and mouldy which I can't accept.

Regarding the leaking heat elsewhere thing, I think the tricky part must be the bearings. I can get the builders to insulate it from the cavity alright but it's got to rest on the outer leaf at some point.

Thanks again that's really given me something to go on, and I'll report back!

 

[dogfonos]

Thanks again that's really given me something to go on, and I'll report back!

Would really like to hear the outcome, John D v2.0, as I'm facing a similar issue.

 

[John D v2.0]

In the end the architect drew a new detail with eps around the front and bottom of the steel isolating it from the outside as much as possible therefore keeping it warm. He said to keep an eye on it next winter as there's still condensation risk.
The builder refused to fix it properly so we have a retention on that until next year.
The ideal solution would have been a vapour barrier on the inside and plenty of wool, keeping the steel cold.

 

[cjard]

there's still commendation risk.

A chance of winning a Drafta award?

Even better if your wife remains upset with it; builder will be getting Victoria cross

 

[John D v2.0]

A chance of winning a Drafta award?

Even better if your wife remains upset with it; builder will be getting Victoria cross

blame auto incorrect! Edited it now...